The present invention relates to an electrode for an electrolytic cell, particularly for electrolytic display cells.
More specifically the present invention relates to an electrode which, compared with presently known electrodes, has an improved resistance to electrochemical corrosion under the operating conditions of an electrolytic cell and specifically under the operating conditions of an electrolytic display cell.
An electrolytic display cell generally comprises an electrolyte containing metallic ions inserted between a first wall coated with an electrode comprising a transparent conductive layer which is suitable for display purposes and a second wall coated with a counter-electrode, as well as electricity supply means connected to the electrode and to the counter-electrode to bring about, by electrochemical reaction from said ions, the deposition or dissolving of a metal coating on said electrode. Thus, the display is obtained when the metal coating whose optical properties (reflection, transmission or absorption) are suitable for bringing about the formation of a mark (character, curve, segment, etc.) is deposited or dissolved on the electrode.
In cells of this type the electrode generally comprises a layer of electricity-conducting transparent material having good electrical conduction and optical transmission properties in the visible range, for example an indium oxide layer.
However, the presently known electrodes of this type have the disadvantage of an inadequate electrochemical resistance, particularly due to the appearance of parasitic secondary reactions which may occur on the electrode during the operation of the cell under the action of overvoltage phenomena. Thus, when using indium oxide transparent electrodes with a silver salt-based electrolyte the parasitic secondary reaction consisting of a reduction of the indium oxide generally occurs when the overvoltage on the electrode is above 800 mV relative to an Ag/Ag.sup.+ silver electrode and leads to significant wear to the electrode after about 200 to 300 operating cycles.
To obviate this disadvantage the transparent electrodes of electrolytic display cells have been made from a material having a better resistance to electrochemical corrosion, for example tin oxide.
However the use of materials of this type leads to other disadvantages. Thus, their electrical conduction characteristics are not satisfactory, they have to be used in the form of much thicker layers which is prejudicial to the transparent qualities of the electrode and due to their corrosion resistance properties they are difficult to etch in a homogeneous manner.